In a WDM optical transmission system, a plurality of optical signals, or channels, are transmitted through a single optical fibre. These channels each have different carrier wavelengths and are separated (de-multiplexed) at a receiving end to produce a plurality of independent received signals. In this way, a large amount of data may be transmitted through a single optical fibre.
During transmission along the fibre, signal quality of each channel will degrade due to fibre losses. Moreover, the signal quality will further degrade if additional components, such as amplifiers, are introduced to the line.
The fibre losses and amplifier gains along a link in an optical transmission network have a dependency on wavelength. Even if all wavelengths in a WDM system are launched with the same power the power levels at the receivers will be different for each wavelength. Furthermore the transmission quality will be different for each wavelength.
In order to address the above problem, the launch powers can have a varying degree of pre-emphasis applied to equalize channel optical signal to noise ratios (OSNRs) at the receivers, or more usefully, equalize the transmission quality of the wavelengths. The latter is more appropriate as received OSNR is not the only factor affecting the relative performance of each wavelength. The pre-emphasis takes the form of varying the drive signal applied to the source of each channel, thereby altering the relative transmit power of the channel.
Some time varying effects in the transmission system, such as polarization induced fluctuations, amplifier aging, system repair, temperature effects, which differentially affect the channels, can be mitigated by adjusting the pre-emphasis applied to each wavelength. However, this mitigation is only useful if the wavelength pre-emphasis is continuously adjusted throughout the life of a transmission system.
In a conventional system arranged to offer pre-emphasis, a central controller is provided to adjust the power output of each transmitter in the system used to transmit the constituent channels of the WDM signal. In this way, the relative optical powers of the constituent channels of the WDM signal can be optimised.
However, such conventional systems are often slow to react to changes in the system for a number of reasons. For example, the calculations required by the central controller in order to calculate the optimal pre-emphasis across the WDM signal are complex. Moreover, in order to optimise the power of each channel, the controller must separately transmit the calculated optimum to each transponder. This causes an inherent delay in the operation of the system. While the extent of this delay may be acceptable in relatively stable systems, the burden on components of the network linking the controller to the transponders during periods of significant change (when a large number of instructions must be sent) is found to result in an unacceptable delay which is prejudicial to the quality and effectiveness of the entire communications system.